1. Field of the Invention
The present invention relates to a microporous polyethylene film and a method of producing the same. More particularly, the present invention pertains to a microporous polyethylene film and a method of producing the same, in which a resin composition suitable for a roll-type stretching in a machine direction is employed, and a two-step biaxial stretching process, where stretching is implemented in a transverse direction using a tenter after stretching is conducted in the machine direction under a condition that stretching is dispersedly conducted, is conducted, thus assuring high productivity, quality consistency, and excellent physical properties. Thereby, the film can improve the performance and stability of a battery using it.
2. Description of the Related Art
Having chemical stability and superior physical properties, a microporous polyolefin film is widely used as various battery separators, filters, and ultrafiltration membranes.
The production of the microporous film using polyolefin may be conducted according to the following three processes. In a first process, polyolefin is processed into a thin fiber to produce a nonwoven fabric-shaped microporous film. A second process is a dry process, in which a thick polyolefin film is prepared and stretched at low temperatures to create micro cracks between lamellas corresponding to crystalline portions of the polyolefin to form micro pores in the polyolefin. A third process is a wet process, in which polyolefin is compounded with a diluent (a low molecular weight organic substance having a molecular structure similar to polyolefin) at high temperatures to form a single phase, phase separation of polyolefin and diluent is initiated in a cooling step, and the diluent is extracted to form pores in polyolefin. In comparison with the first and second processes, the wet process, corresponding to the third process, produces a thin film having uniform thickness and excellent physical properties, and thus, the film produced according to the wet process is widely used for an isolation membrane of a secondary battery, such as a lithium ion battery.
A method of producing a porous film according to a wet process is disclosed in U.S. Pat. No. 4,247,498. This patent discloses a method comprising blending polyethylene and a compatible liquid compound at high temperatures to form a thermodynamically homogeneous phase solution, and cooling the solution to initiate solid/liquid or liquid/liquid phase separation of the polyethylene and the compatible liquid compound, thereby producing the porous polyolefin film. However, the method does not use a stretching process.
In conjunction with the earnest use of a secondary battery, continuous efforts have been made to improve the productivity and physical properties of a microporous film employing a wet process. A representative example uses ultra-high molecular weight polyolefin (UHMWPO) with a weight average molecular weight of about 1,000,000, or mixes such a UHMWPO with a composition so as to increase the molecular weight of the composition, or employs a stretching process so as to improve the strength of the porous film.
With respect to this, U.S. Pat. Nos. 5,051,183, 5,830,554, 6,245,272, and 6,566,012 disclose a method of producing a microporous film, in which a sheet is produced using a composition mixed with polyolefin having a weight average molecular weight of 500,000 or more and a solvent capable of dissolving polyolefin at high temperatures, and the sheet is sequentially subjected to a stretching process and a solvent extraction process. These patents adopt uniaxial stretching or biaxial stretching in the stretching process, and a typical tenter, roll, calendar method, or combination thereof is employed. With respect to the biaxial stretching, the patents give an extensive description, which comprises both simultaneous biaxial stretching and two-step biaxial stretching. However, all of the examples in the patents are limited to simultaneous biaxial stretching, or give explanations only of the biaxial stretching, but do not mention stretching temperatures in machine and transverse directions. In other words, the patents do not disclose a characteristic of the two-step biaxial stretching in which the stretching is conducted in the transverse direction using a tenter after the stretching is conducted in the machine direction using a roll, a characteristic of simultaneous biaxial stretching, or a difference between the two characteristics.
Of commercial microporous polyolefin films, goods which are considered to have excellent physical properties and are created through the wet process are classified into goods produced by conducting stretching after a diluent has been extracted and other goods produced by conducting stretching before a diluent is extracted. In the latter, the stretching is easily achieved because of the softness of the polyolefin, imparted by the diluent. Furthermore, since the film is made thin by stretching, the diluent can be easily removed from the film. However, it is extensively known that, up to now, the production of commercial goods by stretching before extracting the diluent has been accomplished only using the simultaneous biaxial stretching method.
Simultaneous biaxial stretching is a stretching method in which a sheet made from a composition of polyolefin and diluent is fixed using chucks (clamping means) for seizing both ends of the sheet in a form resembling thumbs and index fingers of both hands coming into contact with lower and upper surfaces of the sheet, respectively, and the chucks are simultaneously pulled outward in transverse and machine directions. In this method, since there is holding power when the chucks seize the sheet, slippage does not occur during the stretching, and there are no defects on the surface of a middle portion of the sheet in practice because the stretching is conducted only while both ends of the sheet are seized. Accordingly, the film produced using this method can be used for optical applications.
A portion of the chuck, which seizes the sheet, has the shape of a circle or oval having a diameter of 10-30 mm. The distance in the machine direction between the centers of the chucks is designed so as to be 20-60 mm. If the sheet is stretched in the machine direction by a stretching ratio of 6 times, the distance in the machine direction between the centers of the chucks is 120-360 mm, thus they are spaced apart from each other at an interval from 90 to 350 mm. For example, if circular chucks having a diameter of 10 mm are employed and the distance between the centers of the chucks is designed to be 15 mm so that the stretching is conducted at a stretching ratio of 6 times, since the distance between the centers of the chucks is 90 mm after the stretching, a holding index is just 10/90 (about 11%) based on the machine direction length. Therefore, if the thin film is inspected after stretching, the holding area seized by the chucks is observed to be significantly reduced. However, since there are no chucks on a portion which is not held, no holding power is applied thereto, so it shrinks even though stretching is conducted. Accordingly, there occurs a major disadvantage in that stretching ratios in the transverse direction are different from each other between the portions which are seized and not seized by the chucks. If another stretching ratio is calculated using the same chuck diameter and the same distance between the centers of the chucks as the above example, since the holding index is 10/45 (about 22%) when the stretching is conducted at the stretching ratio of 3 times, and 10/150 (about 7%) when the stretching is conducted at the stretching ratio of 10 times, most portions of the sheet are not held by the chucks after stretching, thus most portions of the sheet shrink in the transverse direction (see FIG. 1).
In addition, since the width of the portion that is not seized by the chuck is narrower than that of the portion that is seized by the chuck, larger portions, on which marks remain due to the chucks and which must be removed in all the stretching processes using the chucks, are cut. This reduces yield during the stretching process. FIG. 1 illustrates the shape of a sheet after it is simultaneously biaxially stretched. Since both edges on which the chuck marks remain must be cut, the effective width of the practically produced sheet corresponds to W. Since the minimum width (L) of the portion of the sheet that shrinks because it is not seized by the chuck is smaller than the distance (H) between the chucks, the effective width (W) of the sheet is reduced, resulting in reduced yield.
Moreover, the chunk used for simultaneous stretching must be designed to endure two-directional stresses simultaneously applied in machine and transverse directions during the stretching, thus it has a complicated structure and a stretching device is costly in comparison with a chuck used to conduct unidirectional stretching. Furthermore, it is disadvantageous in that it provides reduced stretching speed, inconsistent quality, and poor yield due to structural problems with the stretching device.
With respect to physical properties of goods, the most significant disadvantage of the commercial simultaneous biaxial stretching device is that, since the stretching ratio is fixed during a designing stage because of the complicated structure and cost, it is impossible to produce goods requiring the stretching ratio to be changed. For example, when using the simultaneous biaxial stretching device in which stretching ratios are set to 6 times in both machine and transverse directions, it is impossible to conduct stretching so that the stretching ratio is changed to be 5 or 7 times. This means that it is difficult to produce goods having various physical properties in order to meet various needs of consumers.
However, when using a two-step biaxial stretching device, since it is easy to produce goods requiring stretching ratios to be changed in machine and transverse directions, it is possible to produce goods having various physical properties. Additionally, if a microporous polyolefin film is produced using the two-step biaxial stretching device, desirably, it is possible to significantly improve productivity, reduce an installation cost, and reduce defective fractions when working.
Hence, in the production of the microporous polyolefin film using the wet process, if two-step biaxial stretching is employed as the stretching method before the diluent is extracted, the disadvantages of the simultaneous biaxial stretching method can be basically avoided.
In practice, in the two-step biaxial stretching method, it is unnecessary to use the chuck when the stretching is conducted in the machine direction, and the sheet is completely stretched using only a roll, thus there is no problem with respect to the holding. Furthermore, as for the chuck used to conduct the stretching in the transverse direction using a tenter, since the sheet is stretched in the transverse direction using a rectangular chuck, intervals between the chucks are not changed in the machine direction after the stretching is conducted, thus the holding index can be maintained constant before and after the stretching is conducted.
The length of the chuck capable of being used to conduct stretching in the transverse direction is designed to be from 1 to 10 inches. In the chuck designed so as to have a length of 2 inches, the total length of the chuck is 40.8 mm and the interval between the adjacent chucks is 10 mm, thus the holding index is about 80.3% (40.8/50.8×100). In practice, if the holding index is 70% or more after the stretching is conducted, there is no problem with respect to shrinkage of the portion that is not held, thus it is possible to maintain a predetermined stretching ratio through the entire sheet and to minimize the chuck mark area of both edges of the sheet, which is to be removed. Accordingly, it is possible to increase the yield of the stretching process.
Moreover, in the roll and the chuck used in the two-step biaxial stretching, since it is enough to design them so as to endure only one of the stresses occurring in the machine and transverse directions, they have a simple structure and increased holding power in comparison with the chuck used in simultaneous biaxial stretching, thus they are competitive in terms of stability, mechanical operation speed, and installation cost
With respect to the performance of goods, the most important advantage of the commercial two-step biaxial stretching device is that it is possible to produce goods having various physical properties by variously changing the machine direction stretching ratio to be a few times as high as the original stretching ratio, which depends on the number of rolls and constitution of the motor, and that, in the tenter for stretching in the transverse direction, since the stretching ratio can be freely changed depending on the width ranges of an inlet and an outlet, it is possible to produce goods having various physical properties even though only one device is employed.
However, even though the two-step biaxial stretching method is extensively used to produce a thin film, up to now, it has not been applied to the stretching process before extraction in a microporous polyolefin film wet process field. Many reasons may be given for this. Of them, the primary reason is that a slippery sheet which is mixed with an excess amount of organic liquid composition (solvent, plasticizer or the like) must be stretched in the machine direction using a roll. That is to say, in a sheet mixed with an excess amount of an organic liquid compound like oil, since the liquid substance is present on the surface of the sheet, it is difficult to precisely conduct stretching in the machine direction using the roll and a typical stretching method because the sheet slips from the roll. Additionally, since it inevitably comes into contact with the roll during the stretching, the physical properties of the microporous film are deteriorated due to damage to the surface of the sheet caused by frictional force. Particularly, if the sheet is stretched while being pressed using a pinching roll so as to prevent the sheet from slipping, undesirably, the sheet is forcibly made thin and the pore structure of the sheet is deformed and destroyed.
U.S. Pat. No. 5,641,565 discloses a technology of producing a porous polyolefin film, in which an organic liquid compound and an inorganic filler are added to polyolefin to produce a sheet, the organic liquid compound and the inorganic filler are removed from the sheet, and the resulting sheet is stretched. U.S. Pat. No. 5,759,678 discloses that, after a plasticizer is mixed with polyethylene to produce a sheet, the plasticizer is extracted from the sheet, and the resulting sheet is subjected to a simultaneous stretching process or a two-step stretching process to improve strength. Two patents disclose an example with respect to machine direction stretching a using a roll. It is noteworthy that, after the liquid compound or the plasticizer is extracted from the sheet, the resulting sheet is stretched using the roll without slipping. However, since the hard sheet from which the compatible organic liquid compound is removed must be stretched, the stretching ratio of the sheet is reduced, thus breakage easily occurs, the stretching ratio is limited, and defective fractions increase due to small pores. Furthermore, since the sheet which is subjected to an extraction process before the stretching is conducted is thick, a disadvantage, such as reduced extraction efficiency, occurs, as in the production of a microporous polyolefin film through a dry process.
Therefore, the present inventors have conducted extensive studies in order to avoid the problems occurring in the prior art, resulting in the finding that it is possible to produce a microporous polyethylene film having excellent physical properties and productivity, and consistent quality by two-step biaxially stretching a sheet, which contains an organic liquid compound and which is difficult to be processed through a conventional two-step biaxial stretching technology, in such a way that a resin composition suitable for machine direction stretching using a roll is employed, and conditions for the machine direction stretching are controlled.